Regeneration of ferric chloride copper etching solutions

ABSTRACT

ELECTROLYTIC REGENERATION OF FERRIC CHLORIDE COPPER ETCH SOLUTIONS UNDER, ELECTROLYTIC AND CONCENTRATION CONDITIONS EFFECTING FORMATION OF CHLORINE ATOMS AT THE ANODE FOR OXIDATION OF FERROUS IONS IN THE SOLUTION TO FERRIC IONS.

United States Patent 01 fice 3,794,571 Patented Feb. 26, 1974 3,794,571REGENERATION OF FERRIC CHLORIDE COPPER ETCHING SOLUTIONS Stanley J.Beyer and Robert M. Lukes, Louisville, Ky., assignors to GeneralElectric Company No Drawing. Filed May 10, 1971, Ser. No. 142,065 Int.Cl. C22d 1/16; C01g 47/10 US. Cl. 204-94 3 Claims ABSTRACT OF THEDISCLOSURE Electrolytic regeneration of ferric chloride copper etchsolutions under electrolytic and concentration conditions effectingformation of chlorine atoms at the anode for oxidation of ferrous ionsin the solution to ferric ions.

The present invention relates to the regeneration of ferric chlorideetchant used for the etching of copper in the manufacture of printedcircuit boards, printing rolls and the like.

Ferric chloride is probably the most widely used etchant for suchpurposes. The etching reaction involves the oxidation of metallic copperby the ferric ion, the products of the reaction being ferrous ions andcupric ions. When the ferric ion content of the solution becomes too lowfor adequate etching action, it has been a common practice to dispose ofthe solution and substitute a fresh etchant bath. Because of thepollution problems resulting from the disposal of the solutions, thedisposal thereof has become a substantial problem.

It has been proposed to chemically regenerate the etching solutions byoxidation of the iron with hydrogen peroxide or with chlorine gas andthe removal of dissolved copper by precipitation. None of theseapproaches have been in wide spread use, probably because the cost ofthe chemicals or the batch regeneration are greater than the value ofthe regenerated bath.

The electrolytic regeneration of the spent or partially spent solutionshas also been proposed. For example, an elaborate and rather expensiveelectrolytic regeneration cell for this purpose is disclosed in US. Pat.2,748,071- Eisler. In this cell, the cathode is enclosed in aporouswalled chamber which separates the anolyte and catholyte sectionswith the results that the liquid etchant must eventually pass throughthe porous walls which in a short time become clogged with suspendedsolids such as the insoluble or slightly soluble copper or ironcompounds.

To avoid these problems and to dispense with porous walls, Pat. No.2,964,453Garn et al. specifically describes the use of a cupric chloridebath which according to the patentees can be easily regeneratedelectrolytically employing an anode-to-cathode ratio such that thecurrent density is high at the cathode and is low at the anode. Thiscathode-to-anode ratio is primarily employed so the metallic copper willbe deposited on the cathode by a cathode potential great enough toreduce cuprous ions to metallic copper. The Garn et al. cupric chloridebath must also include an excess of chloride ions which function tostabilize the cuprous valence state in solution and favor a reaction inwhich cupric ions oxidize copper to the cuprous state and are themselvesreduced to the cuprous state. Due to the large anode-to-cathode ratiorequired in carrying out the Garn et al. process, proportionately largecells or anode configurations are required. -In addition the requirementfor an excess of chloride ions increases the chemical cost of thesolutions. The Garn et al. patent also suggests, without elaboration,that a ferric chloride bath could be used in place of the cupricchloride bath consistent with the spirit of the Garn et al. in-

ride as suggested by Garn et al. in a regeneration cell free of a porousseparator wall or diaphragm does not provide an eflicient low cost andeffective method for regenerating the commonly used ferric chloridecopper 5 etching baths. More specifically, an electrolyticallyregenerating cell designed consistent with the spirit of the Garn et al.invention requires a relatively large anode area, because regenerationis based on an anode-to-cathode area ratio of at least three andpreferably larger than five. The cell itself must therefore be large toprovide a large anode area. Regeneration under these conditions islimited to the direct oxidation of the ferrous ion at the anode, whichin turn requires a substantial concentration of ferrous ion in the spentsolution.

The present invention does not require a high anodeto-cathode area ratioand, in fact, can be operated with equal anode and cathode areas. Alsoit does not require a high concentration of ferrous ion, or its directoxidation at the anode for regeneration. Instead, our inventionaccomplishes regeneration first by forming an active species of chlorinein situ through oxidation of chloride ion at the anode, and thenpermitting this active species to oxidize ferrous ion to ferric ion inthe bulk of the solution. Thus oxidation of ferrous ion to ferric can beeffected even at low concentration of ferrous ion, an achievement notpossible under the teachings of Garn et al.

With the present invention, there is no visible formation or liberationof gaseous chlorine, and oxidation of ferrous ion in this manner is notthe same as oxidizing by adding gaseous chlorine. By utilizing chlorideions already in solution, there is no gaseous chlorine formed, thechloride ions are used over and over, and the chlorine as formed at theanode is much more active than chlorine gas.

The etching solution to be used with a cell that does not require a highconcentration of ferrous ion is not bound to use ion concentration whichdevelop from an etching solution which begins as ferric chloride andhydrochloric acid and acts upon metallic copper. Instead, all ionconcentrations can be controlled independently.

Contrary to the teachings of Garn et al., the subject iron chlorideetching solution does not require an excess chloride concentration tostabilize or solubilize other ionic species, as for example cuprous ion.However, excess chloride ion is in no way detrimental to the operationsof either regeneration of ferric ion or dissolution of copper.

As stated before, the various ion concentrations that can be utilizedwhile practicing the invention are relatively independent of each other.Preferably, the following concentration limits should be observed:

If for some reason not connected with the process of regeneration offerric ion, one wishes to use a concentration of chloride ion in excessof that required to balance the concentration of Fe+++, Fe++, and Cu++,then the addition of other inert metallic chlorides is acceptable;provided, however, that such addition is consistent with solubilityrequirements. Such chlorides include but are not limited to MgCl CaClNaCl, KCl, BaCl, LiCl, ZnCl and HCl.

As discussed above, the teachings of Garn et al. require ananode-to-cathode area ratio of at least three. The present inventiondoes not have this limitation,

operating at lesser anode-to-cathode area ratios. For best operation,the following limits are preferred:

High limit Low limit Anode/cathode area ratio 2:1 Anode current density(amperes/sq. it.) l, 000

Thus electrons are transferred from the copper to the ferric ion in theprocess of dissolving the copper. In time, the reduction of theconcentration in the etching solution slows the etch rate and finallydissolution of copper stops. When the etchant is about 85% spent, itsability to dissolve copper slows drastically and the etch solution isdiscarded.

In conventional practice, wherein one starts with 2.25 M or 3.75 m FeClthe concentrations of Cu++, Fe+++ and Pe at any one time are highlyinterdependent, and change as the etching process proceeds. Furthermore,the etch rate is highly dependent upon the relative concentrations ofthese ions.

With the present electrolytic regeneration process, the opportunityexists to formulate an etch solution containing the desiredconcentrations and ratios of Cu++, Fe+++ and Fe++ to get a desired etchrate, and to maintain these ratios and concentrations by continuousregeneration and copper removal to balance the copper dissolution.

Rapid and efficient copper removal from solution by plating requires arelatively high Cu++/Fe+++ ratio. This can be achieved without loweringthe concentration of Fe+++ below the level required for an adequate etchrate, since the Cu++ and Fe+++ concentrations can be set independently,as described above.

Continuous removal of copper metal requires a special cathode design.Such designs include, but are not limited to:

(1) A moving metal belt cathode that passes through the solution, andfrom which copper is removed by a doctor blade or other mechanicalmeans.

(2) A rotating drum cathode, the axis horizontally oriented, from whichthe copper is removed continuously by a doctor blade or other means.

(3) A moving chain of cathode plates, only a fraction of which areimmersed in the solution at any one time, and from which copper isremoved mechanically or chemically external to the solution.

(4) Use of the tank itself as the cathode, and remove copper powder as asludge from the bottom continuous 1y.

(5) Removable cathodes that are exchanged periodically and are strippedof copper in a separate operation.

Since cathode designs of these types are well known, many of them beingshown and described in the aforementioned Garn et al. patent, no furtherdiscussion thereof is believed necessary.

Under certain circumstances, as in making printing rolls for the gravureindustry, other requirements for the solution may supervene. Inpreparing such gravure rolls, the etching of copper is done thrOugh aresist of gelatin or other semipermeable material. Thus the desiredetching characteristics of the ferric chloride solution are a functionnot only of the Fe+++, Fe++ and Cu concentrations, but also of thesolution viscosity and osmotic pressure, as well as the diffusion andpermeation rates of the virgin and spent solutions into and out of thesemipermeable resist.

For such purposes, monovalent or divalent metal chlorides can be addedto the etch solution to adjust viscosity, osmotic pressure and totalionic strength. For example, calcium chloride, magnesium chloride orbarium chloride could be used without interfering with the electrodeprocesses.

In this way one can set the Fe+++, Fe++ and Cu++ ionic concentrationsfor optimum etch rate and regeneration, and then add the inert metalchlorides to adjust the viscosity, osmotic pressure, diffusion rate andpermeation rate of the solution.

Because oxidation is accomplished by direct electrolytic generation ofchlorine at an anode, it is essential to have a minimum quantity ofchloride ion present in the solution. Chloride ions may be supplied inpart by metal chlorides other than iron chlorides (e.g. calcium,magnesium or barium chloride) to obtain a viscosity and osmotic pressuresuitable for etching copper through a semipermeable resist or membraneof the types used in etching gravure cylinders or by iron chloridesalone as may be desirable for etching where resists are not permeable(e.g. printed circuit boards).

Where semipermeable resists are used, the basic iron chloride solutioncan also be modified with 0.1% to 10% of an organic nonionic Watersoluble polymer such as polyethylene oxides, carboxymethylcellulose, orhydroxyethylcellulose.

Within the above limits there are, of course, certain preferredcompositions. From the standpoint of chlorine generation, highconcentration of chloride ion is favorable. The presence of ferrous ionappears to be inconsequential. If ferrous ion is present in highconcentrations, it becomes oxidized; and ferric ion becomes regenerateddirectly. If ferrous ion is present in low concentrations, it becomesoxidized indirectly by the generation of chlorine and subsequentchemical reaction with chlorine to produce ferric chloride:

From the standpoint of the electrolytic removal of copper from the ironchloride solution, high concentrations of copper are desirable.

The amount of ferric ion present in the etching solution is important.High concentrations are unfavorable in that cathode current efficiencyfor electrolytic deposition of copper is reduced with increasing ferricion concentration. More and more electron transfer at the cathode isaccomplished by the reduction of ferric ion rather than copper ion asthe concentration of ferric ion increases. On the other hand, ferric ionis required to obtain a satisfactory etch rate.

The balancing of etch rate and regeneration eificiency will be a matterof preference which may vary depending on the requirements of the user.However, a preferred point of control of electrolyte composition is asfollows:

FeCl 1.0 M FeCl 2.5 M CuCl z 1.0 M

with -0 to 1.0 M of other chloride and 0 to 10% organic polymer addedonly to affect the etch rate through a particular semipermeable resist.

Electrolytic regeneration can be accomplished in the same vessel usedfor etching, but it is preferable to circulate the etching solutionthrough a separate electrolytic cell, because solution flow past theelectrodes carries away the heat generated and the products ofelectrolytic action, while supplying fresh chloride ion to the anode andcopper ion to the cathode. Also the solution can be more readilymonitored using a reference electrode to initiate and stop electrolyticregeneration when the solution is recirculated.

Electrolytic removal of copper from the etching solution can be obtainedusing cathode current densities of As previously indicated, thedeposition, removal, and

recovery of copper can be accomplished in several ways. One method is touse a slowly rotating drum cathode immersed about one-third into theetch solution. Rotation is such as to provide about two minutes ofelectrodeposition at any point on the circumference of the drum. Thedrum surface should be constructed of a metal which is not chemicallyattached by the etch solution and does not receive an adherent copperdeposit (e.g. tantalum). A doctor blade is used to scrape the looselyadherent powdered copper from the rotating drum. The powder will thendrop to the bottom of a running water rinse tank onto a moving beltconveyor, through a drying oven, and into a collecting container.

Another method of recovering copper is to use a moving metal beltcathode which conveys the copper powder on its surface through rinsingand drying. The dried copper powder can than be scraped off into acollecting container. A modification of this procedure is to pass themoving belt through an acid copper or other copper plating solution asan anode and collect the copper as pure copper cathode sheet.

A third method for recovering copper is to provide a conveyor ofseparate moving cathodes. As one cathode is conveyed out of the etchbath, another cathode enters to maintain a given cathode area. Thecathodes are then conveyed through water rinses and then pass through acopper plating solution as anodes to recover the copper as pure cathodesheet.

The method of removing and recovering copper is incidental. What isimportant is that the etch solution be controlled within the aboveprescribed chemical concentration limits so that good cathode efliciencyfor depositing copper is obtained and the bath retains a standardetching capability only through the passage of electric current throughan electrolytic cell rather than using fresh ferric chloride solutionuntil it is nearly spent, discharging the solution, and making it freshagain as is current industry practice.

While there has been shown and described certain embodiments of thepresent invention, it is obvious that it is not limited thereto and itis intended in the appended claims to cover all modifications fallingwithin the spirit and scope of the invention.

What we claim is:

1. The method of continuously regenerating a ferric chloridecopper-etching solution containing ferrous ions during use of thesolution for copper etching which comprises electrolytically maintainingthe ion concentrations of said solution within the limits of 0.7 to 2 Mferric ion; 0 to 3 M ferrous ion, 0.2 to 1.5 M cupric ion and 2.5 to11.5 M chloride ion employing an anode-to-cathode ratio of 2:1 to 0.5:1and an anode current density of from 25 to 1000 amperes per square footwhereby copper is plated from said solution at the cathode and chlorideions are oxidized at the anode for oxidation of ferrous ions in saidsolution to ferric ions.

2. The method of claim 1 in which the copper deposited on said cathodeis at least periodically removed therefrom.

3. The method of claim 1 in which said anode is composed of graphite.

References Cited UNITED STATES PATENTS 272,391 2/1883 Thiollier 2041063,394,060 7/1968 Douglas 20494 1,969,678 8/1934 White et al 156-192,886,420 5/1959 Jones et al. 156-19 3,083,129 3/1963 Jones et al.156-19 OTHER REFERENCES Regenerable Etchant for Copper by Sharpe et al.,Ind. & Eng. Chem., vol. 51, No. 3, pp. 293-298, March 1959.

RCA Tech. Notes by Custman, 2 pp., pub. March 1970.

F. C. EDMUNDSON, Primary Examiner U .5. Cl. X.-R. 15619

